Scientists at the Universities of Bristol and Southampton have developed an important new insight into climate sensitivity — the sensitivity of global temperature to changes in the Earth’s radiation balance — over the last half million years. Climate sensitivity is a key parameter for understanding past natural climate changes as well as potential future climate change. In a study in Journal of Climate, the researchers reconstructed, for the first time, climate sensitivity over five ice-age cycles based on a global records of sea surface and polar temperature change. These were compared with a new reconstruction of changes in the Earth’s radiation balance caused by changes in greenhouse gas concentrations, in surface reflectivity, and those due to slow changes in the Earth-Sun orbital configuration.

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The study calculates global mean climate sensitivity, but also considers its relationship with latitude. This is important as many of the past radiative changes were not equally distributed over the planet in contrast to the more uniform distribution of the modern radiative changes due to rising greenhouse gas levels.

Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate. All climate models balance, or very nearly balance, incoming energy as short wave (including visible) electromagnetic radiation to the earth with outgoing energy as long wave (infrared) electromagnetic radiation from the earth. Any imbalance results in a change in the average temperature of the earth.

The researchers infer that the Earth’s climate sensitivity over the last half million years most likely amounted to a 3.1 to 3.9 °C temperature increase for the radiative equivalent of a modern doubling of atmospheric carbon-dioxide concentrations (with a total range of 1.7 to 5.7 °C).

Lead researcher Eelco Rohling, Professor of Ocean and Climate Change at the University of Southampton, said: “We use long time-series of data that are each obtained using a single method. This reduces uncertainty in the estimates of temperature change, relative to previous work that contrasts reconstructions of a single past climate state with modern instrumental data. Our method can be especially improved by extending the global network of long records.

"Because our climate sensitivity values are based on real-world data from a substantial interval of time in the recent geological past, our results provide strong observational support to the climate sensitivities used in IPCC-class climate models. If anything, our results suggest slightly stronger sensitivity."

Dr Mark Siddall, from the School of Earth Sciences at the University of Bristol, said: "This study shows the increasing importance of using geological data to understand the climate system and how it responds as a whole to changes in greenhouse gases."